The present invention relates generally to a telemetry system for transmitting data from a downhole drilling assembly to the surface of a well during drilling operations. More particularly, the present invention relates to a mud siren pressure pulse generator for use in a measurement while drilling ("MWD") system or a logging while drilling ("LWD") system to transmit downhole measurements to the surface of the well during drilling operations through the medium of the drilling fluid. Still more particularly, the present invention relates to a self-propelled mud siren for accurately and efficiently transmitting downhole drilling or borehole information to the surface.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde is supported by a conductive wireline, which attaches to the sonde at the upper end. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
More recently, those in the industry have placed an increased emphasis on the collection of data during the drilling process. By collecting and processing data during the drilling process, without the necessity of removing or tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections, as necessary, to optimize performance. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
There are many systems available for transmitting data indicative of downhole parameters to the surface during the drilling of a well. One early system is that disclosed in U.S. Pat. No. 3,309,656, which used a downhole pressure pulse generator or modulator to transmit modulated signals, carrying encoded data, at acoustic frequencies to the surface through the drilling fluid or drilling mud in the drill string. In this and similar types of systems, the downhole electrical components are powered by a downhole turbine generator unit, usually located downstream of the modulator unit, that is driven by the flow of drilling fluid.
Prior art mud siren modulators typically take the form of turbine-like signal generating valves positioned in the drill string near the drill bit and exposed to the circulating drilling fluid. In many instances, the modulator assembly is comprised of a fixed stator and a motor-driven rotatable rotor, positioned coaxially with respect to each other. The stator and rotor usually are formed with a plurality of radial lobes spaced circumferentially around a central hub, so that the gaps or ports between adjacent lobes provide a plurality of openings through which the drilling fluid may flow. When the respective ports of the stator and rotor are directly aligned, the area for fluid flow through the modulator is at a maximum. As the rotor rotates with respect to the stator, and the lobes are no longer in alignment, the flow of drilling fluid is restricted, which generates pressure pulses, in the form of acoustic signals in the column of drilling fluid. As the rotor is continuously rotated with respect to the stator, a cyclic acoustic signal is produced that travels up the drilling fluid column and which is detectable at the surface of the well by the use of acoustic transducers. By selectively varying the rotation of the rotor, changes in the acoustic signal can be achieved, enabling modulation in the form of an encoded pressure pulse that can carry information indicative of downhole parameters to the surface for immediate analysis.
Depending upon whether the rotor is positioned upstream or downstream with respect to the stator greatly affects the tendencies of the rotor. The placement of the rotor upstream from the stator subjects the rotor to fluid dynamic forces due to the fluid stream that generally causes the rotor to seek a stable closed position, in which the lobes of the rotor block the ports of the stator to inhibit fluid flow through the modulator. Thus, it has been found that in this configuration, the rotor will assume a position that blocks the flow of drilling fluid whenever the rotor or the motor driving the rotor becomes inoperable. This tendency increases the likelihood that the modulator assembly will jam, as solids carried in the fluid stream are forced to flow through restricted passages in the modulator assembly. In addition, restarting the rotor is more difficult because the reduced mud flow through the modulator assembly directly affects the generation of power by the mud turbine, which is located downstream from the modulator. Prolonged modulator closing can obstruct mud flow to such an extent that lubrication of the drill bit, and other vital functions of the drilling mud, become so adversely affected that the entire drilling operation is rendered ineffective, and may even result in serious damage to the components of the bottom hole drilling assembly.
A number of methods have been investigated to overcome the problem caused by the tendency of modulator assemblies to assume a closed position. One approach, suggested for example in U.S. Pat. No. 3,792,429, is to use a magnetic force to bias the modulator assembly to an open position in the event that the rotor becomes inoperative. Magnetic attraction between a magnet attached to the modulator housing and a cooperating magnetic element positioned on the rotor shaft is used to overcome the fluid dynamic torque caused by the drilling mud stream. This method, however, has several disadvantages. First, the modulator assembly must be extended in length to accommodate the magnets. Second, the introduction of an extraneous magnetic field downhole may potentially interfere with simultaneous measurements of the earth's magnetic field, which commonly is used to derive tool orientation.
Another method is to alter the spacing between the rotor and stator based upon the speed of the rotor. Typically, the rotor and stator are spaced very closely together to produce satisfactory acoustic signals, thus increasing the likelihood that debris in the drilling mud will become jammed or lodged in the modulator assembly. As disclosed in U.S. Pat. No. Re. 29,734, a control device is used that senses parameters indicative of the rotor slowing, such as an increase in pressure differential across the modulator assembly or an increase in the motor torque that drives the rotor. In response to these indicia of the rotor slowing, the control device temporarily separates the rotor and stator in an attempt to clear the debris from the modulator assembly by the flow of drilling mud.
A third approach is to switch the position of the stator and rotor, as suggested in U.S. Pat. No. 4,785,300, to change the tendency of the modulator assembly to assume a closed position. Placing the rotor downstream from the stator changes the stable state of the modulator assembly from a closed position, in which the lobes of the rotor align with the ports of the stator, to an open state, in which the lobes of the rotor align with the lobes of the stator. In accordance with this method, the lobes of the rotor are specially designed with an outwardly tapered configuration to enhance this effect. Because this modulator assembly assumes an open position in the absence of power to the rotor, there is less of a chance that debris will become lodged in the modulator assembly. Despite this improvement, however, and because the rotor still exhibits an inherent tendency to "freeze" (albeit, in the open position), the prior an invention disclosed in U.S. Pat. No. 4,785,300 still may be subject to debris lodging in the narrow area between the stator and rotor when the rotor ceases to rotate, causing the modulator assembly to jam when power is resumed to the rotor.
To date, no one in the industry has successfully developed a modulator assembly for a mud siren with a rotor that has an inherent tendency to continue to rotate as drilling mud flows through the modulator. Similarly, no one has developed a self-generating mud siren modulator to eliminate the necessity of a separate motor to drive the rotor, despite the apparent advantages inherent in such a design.